1. Field of the Invention
This invention relates to devices and methods that use OTDR measurement for testing of multi-branch optical networks which branch off by optical lines.
This application is based on Patent Application No. Hei 10-60131 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
FIG. 10 is a block diagram showing an example of a system configuration for a multi-branch optical network testing device, which is conventionally known. The multi-branch optical network testing device of FIG. 1 is designed to perform a fault isolation test on an optical network of eight-branch type, which is provided in a 1.31/1.55 wavelengths multiplex transmission system. In FIG. 1, an OTDR measurement device 1 (where xe2x80x9cOTDRxe2x80x9d is an abbreviation for xe2x80x9cOptical Time Domain Reflectometerxe2x80x9d) outputs test beams of 1.6 xcexcm band, which are incident on an optical line 3 via an optical coupler 2. Then, the test beams branch out by a star coupler 4, from which they are distributed to optical fibers fb1 to fb8 respectively.
The optical fibers fb1 to fb8 are respectively connected to ONUs (i.e., Optical Network Unit or Subscriber Network Device). On the optical fibers fb1 to fb8, filters 41 to 48 are provided prior to the ONUs respectively. Each of the filters 41 to 48 has a band-pass characteristic in which only a signal beam corresponding to each of the ONUs is allowed to pass therethrough while the test beam is reflected thereby. Therefore, the test beams propagating through the optical fibers fb1 to fb8 are respectively reflected by the filters 41 to 48, so that reflected test beams (simply called reflection beams) propagate backwardly through the optical fibers 41 to 48 respectively. Those reflection beams are subjected to wave mixing while being passed through the star coupler 4, which thus produces response beams. Then, the response beams are returned to the OTDR measurement device 1. Thus, the OTDR measurement device 1 analyzes the response beams.
FIG. 11 is a graph showing an example of waveforms of the response beams which are observed by the OTDR measurement device 1. The waveforms show time-series variations of the response beams. In FIG. 11, a horizontal axis represents a value which is produced by multiplying propagation time of the response beam by transmission speed of light, that is, a length of the optical fiber that the response beam propagates through.
The response beams are produced by mixing the reflection beams reflected by the filters 41 to 48 respectively. Those filters are located at different positions on the optical fibers fb1 to fb8 with different distances from the OTDR measurement device 1 respectively. For this reason, the reflection beams, which are reflected by the filters 41 to 48 respectively and which are observed by the OTDR measurement device 1, do not overlap with each other on the time axis, so they are observed in a separate way. A waveform R shown in a leftmost area of the graph of FIG. 11 corresponds to a reflection beam from the star coupler 4. Waveforms, which follow the waveform R and which are sequentially arranged in the graph from the left to the right, correspond to the reflection beams which are respectively reflected by the optical fibers fb1 to fb8 and which are returned to the OTDR measurement device 1.
FIG. 12A and FIG. 12B are graphs showing magnified images of waveforms of the response beams corresponding to the reflection beams which are output from the optical fibers fb6 to fb8 respectively and are observed by the OTDR measurement device 1.
The graph of FIG. 12A is made with respect to a non-fault situation where no fault occurs on any of the optical fibers fb6 to fb8, while the graph of FIG. 12B is made with respect to a fault situation where a fault is simulated by imparting a bend loss of 3 dB to the optical fiber fb7.
It is understood from the graphs that reduction occurs in intensity of the reflection beam with regard to the optical fiber fb7 on which a fault is simulated.
According to the system configuration shown in FIG. 10, it is possible to detect the fault that occurs on the optical network by analyzing intensities of the reflection beams corresponding to the response beams returned to the OTDR measurement device 1.
Incidentally, the aforementioned technology is disclosed by the paper B-846 entitled xe2x80x9c1.6 xcexcm-band Fault Isolation Technique For Passive Double Star Networksxe2x80x9d, which is issued in 1994 autumn meeting of the Institute of Electronics, Information and Communication Engineers of Japan.
However, the aforementioned multi-branch optical network testing device suffers from problems as follows:
i) The aforementioned multi-branch optical network testing device is capable of specifying the optical line on which the fault occurs. However, it is impossible to detect a distance of a fault occurrence point (or position) on the optical line.
ii) The multi-branch optical network testing device requires the facilities that the filters are arranged respectively on the xe2x80x9cbranchedxe2x80x9d optical fibers at different positions by which intervals of distance measured from the coupler and filters differ from each other. This brings a limitation in fiber lengths.
iii) The multi-branch optical network testing device requires the optical fibers to have filters respectively. This requires high cost for construction of the system.
It is an object of the invention to provide a multi-branch optical network testing method and a multi-branch optical network testing device, which are capable of automatically detecting fault occurrence times, fault occurrence lines and fault occurrence distances with regard to multi-branch optical networks.
A multi-branch optical network testing method (or device) of this invention is provided to perform a fault isolation test on an optical network that branches off at a branch point (e.g., optical coupler) by a number of optical lines having terminal ends respectively. Herein, optical pulses are input to the optical network, from which they are returned as reflection beams. Then, response beams corresponding to mixture of the reflection beams are converted to OTDR waveform data representing a waveform whose optical power gradually decreases in accordance with a distance from an OTDR measurement device and which has a number of reflection peaks.
The OTDR waveform data are subjected to logarithmic conversion to produce logarithmic waveform data representing a logarithmic waveform. An approximation method of least squares is effected on the logarithmic waveform data to produce an approximation line, which crosses the logarithmic waveform at points of intersection corresponding to Fresnel reflection points. Using the Fresnel reflection points as split points to split the OTDR waveform data into a number of ranges. Attenuation constants are repeatedly calculated with respect to each of the ranges every measurement time and are stored in a storage device.
Thereafter, fault determination is automatically performed based on the attenuation constants stored in the storage device with respect to the fault occurrence time, fault occurrence line and fault occurrence distance. Herein, the fault determination is made in response to a change that occurs between the attenuation constants sequentially calculated at the consecutive measurement times with respect to each of the ranges.